This invention relates to subtraction fluoroscopy systems. In particular, the invention is an improvement in performing logarithmic compression of video signals in such systems.
In single x-ray images of an anatomical region, bone, blood vessels or other tissue of primary diagnostic interest may be surrounded or overlayed or underlayed by tissue or bone which obscures and prevents visualization of the fine structure of interest. Subtraction of two images of the same region is performed to remove or suppress the potentially confusing effects of uninteresting overlaying and underlying structures to thereby enhance the detectability and display of the structure of interest.
An important use of the image subtraction process is making angiographic examinations, that is, making x-ray examinations of blood vessels, typically the arteries of the heart or other organ. In this technic, a fluoroscopic x-ray image of the heart or other organ is made and converted to corresponding digital picture element (pixel) values. Shortly after one image is made, an x-ray opaque medium, such as an iodine compound, that has been injected into the blood vessels reaches the heart blood vessels and then another x-ray image is made and converted to digital pixel values. When one of the images is subtracted from the other, digital difference pixel signals result. The difference signals are amplified and converted to analog video signals which are fed to a television monitor that displays the difference image. In the difference image, some anatomical structures are deemphasized and a higher contrast and more easily visualized image of the iodine-infused vessels remains.
Methods and apparatus for performing digital x-ray image subtraction are described in U.S. Pat. Nos. 4,204,225 and 4,204,226. The systems shown in these patents produce the successive images for subtraction with a relatively broad spectrum x-ray beam. The x-ray images are converted to optical images with an x-ray image intensifier. The optical images are received by a video camera that converts them to an electrical video signal. This signal is suitably processed and subjected to analog logarithmic amplification before being converted to a series of digital words.
Because attenuation of the x-ray beams by the body is an exponential function of the product of the coefficient of absorption and the thickness of the anatomy in the beam, there is a non-linear relationship between the analog video signals and the corresponding tissue thicknesses. Hence, in digital image subtraction systems, it is desirable to subject the video signals in either analog or digital form to logarithmic amplification. As indicated earlier, an analog logarithmic amplifier can be used before digitization. It has been observed, however, that even if the most fully compensated, broadest bandwidth and highest grade analog logarithmic amplifiers are used, conversion accuracy is not as good as it could be for digital fluoroscopy or image subtraction purposes. Since the input signal is an exponential function of the product of the coefficient of absorption and thickness of the anatomy, if accurate logarithmic transformation occurred, the output signal of the amplifier should be very linear and consistent with a true exponentially decaying input. Even with the highest grade fully compensated analog logarithmic amplifier, the function may not be truly logarithmic and is subject to drift of both the log curve and the dc offset. Wide dynamic range and wide band log amplifiers are very difficult to design or procure. Slight departures from perfect log conformity may be tolerable for difference imaging on a qualitative basis, but it is not acceptable where quantitative digital fluoroscopic work is intended. For quantitative work, accurate logarithms of video picture element signals which do not drift during a patient study are required.
Another problem with analog logarithmic amplifiers is limited bandwith. As dynamic rangge increases, smaller signals are used. Because the gain applied to such signals is large, the bandwidth suffers. Hence, a serious disadvantage of video frequency analog logarithmic amplifiers, particularly at large dynamic range, is limited bandwith.
Another prior art approach is to perform a logarithmic transformation after the video signals have been digitized. A microprocessor or computer may be used to make the transformations. Considering that the digitized video signal frequency is commonly around 12 MHz, the computer would have to make 12 million conversions per second in order for the system to operate in and display difference images in real-time. It has been found that the times for software to transform digital pixel signals to logarithmic equivalents are on the order of one microsecond or more per pixel. So even with the fastest microcomputer and software, conversion has been found to be too slow by a factor of about 10. Slowness results from the fact that an algorithm must be used which re-expresses the logarithm as a power series of additions and multiplications.